This invention relates to a homogeneous process for the conversion of a formic acid ester or a transesterification product thereof to a carboxylic acid. More particularly, the formic acid ester or transesterification product is contacted with an iridium salt and an iodide promoter in the presence of an organic solvent containing carboxylic acid.
The direct conversion of formic aicd esters to carboxylic acids is known. U.S. Pat. No. 3,839,428 which was reissued as Re. 29,981 teaches a process for converting formic acid esters to the corresponding carboxylic acids which comprises contacting the ester with carbon monoxide (CO) at an elevated CO pressure. While the reaction will occur in the absence of a catalyst, Group VIII metals such as iron, cobalt and nickel, Group IIb metals and halogens may be advantageously employed to increase the conversion rate. U.S. Pat. No. 4,194,056 relates to a process for the preparation of acetic acid which comprises heating methyl formate in the presence of a soluble rhodium salt catalyst, halogen promoter and sufficient amounts of carbon monoxide to convert the rhodium salt into at least a monocarbonyl compound. Other metal salts such as cobalt iodide, nickel iodide and rhenium pentacarbonyl were listed as non-catalysts. Carbon monoxide is stated to be essential. British Patent No. 1,286,224 and German Offen. 2,109,025 disclose a process for producing acetic acid comprising heating methyl formate in the presence of carbon monoxide and a catalyst system containing rhodium and a halogen promoter.
U.S. Pat. No. 3,798,267 is directed to the isomerization of methyl formate to acetic acid in the presence of carbon monoxide and catalyst system consisting essentially of activated carbon and halogen promoter. F. J. Bryant et al., preprints Div. of Petr. Chem., 18i 193 (1973) describe the reaction mechanism for the conversion of methyl formate to acetic acid using carbon monoxide, a soluble rhodium complex and methyl iodide at 200.degree. C. The reaction occurs whether or not acetic acid is initially present. U.S. Pat. No. 2,508,513 relates to the conversion of methyl formate to acetic acid. The catalysts are carbonyl forming metals or compounds plus halogens. Such catalysts are iron metals, tungsten, vanadium, antimony and bismuth. U.S. Pat. No. 2,739,169 discloses the reaction of CO and H.sub.2 O with olefins, alcohols, ester and ethers to produce acids. Catalysts are stated to be metal carbonyls with typical carbonyl-forming metals being Ni, Co, Fe, Cr, Mo, Ru, Pd, Pt, Re, Os and Ir.
Japan Kokai 76/65,703 (Chem. Abstr. 85,93872d) discloses that a mixture of acetic acid and methyl acetate is produced when methyl formate is heated with rhenium catalysts, halides and carbon monoxide. Japan Kokai 73/19,286 (Chem. Abstr. 79,78102k) relates to a hydroformylation reaction wherein methyl formate is heated with a CO/H mixture in N-methylpyrrolidone using CoI.sub.2, FeI.sub.3, RuI.sub.3, RhCl.sub.3 and bis(butylpyridinium) tetrabromocobaltate as catalysts to produce aldehydes.
U.S. Pat. No. 3,488,383 concerns the selective decomposition of formic acid in admixture with another aliphatic acid or ester of an aliphatic acid, e.g., methyl formate. The process comprises contacting the formic acid containing mixture with a soluble complex compound of a Group VIII metal or rhenium. The complex compound is a compound of Pt, Os, Rh, or preferably Ru or Ir. Formate esters are unreactive and may be present as part of the solvent system. Example 1 indicates that at least part of the formic acid is decomposed to H.sub.2 and CO.sub.2.